Month: October 2023

Week 4 – How to rotoscope?

Rotoscoping is the technique of manually creating a matte for an element on a live-action plate so it may be composited over another background. Via rotoscoping we can creating animated alpha channels “matte” to match the footage motion(Rotoscoping is a core skill for a digital compositor).

Basic nodes for Rotoscoping

• Roto Node: The fundamental tool for rotoscoping. You can add paths matching on the object. To create a Roto node, press O.
• RotoPaint Node: This node allows for painting directly on the footage, facilitating the rotoscoping process. Using multiple RotoPaint nodes can be heavier compared to Roto nodes.
• Shuffle Node: The Shuffle node is vital for manipulating input and output channels. It is particularly useful when you need to remove an alpha channel. Simply disconnect the alpha channel from the output layer.
• Remove Node: This node empowers you to select the specific channel you want to remove.

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Tips

Thumbnail Channel Information: Under the thumbnail image, you can find a list of channels contained within each source.

I/O Points: Adjacent to the play button, the I/O button allows you to set intro and outro points within your composition.

Calculate motion blur in Nuke: 24 frames per second, 1/60 shutter speed > 24fps/60=0.4

Utilize the ‘Ripple’ function to fix the position of a specific point. Turn it on when making adjustments and off when you’ve completed the rotoscoping.

Press ‘Q’ to toggle the overlay(outline of roto path) on and off.

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When you use Tracker node, firstly add a track point and analyze it. Afterwards, copy transform data by ‘Ctrl+drag’ to the Roto Root transform.

Employ nodes like ‘EdgeDetect’ to create an outline of alpha, ‘Dilate’ to reduce or expand the alpha, and ‘FilterErode’ for alpha manipulation using filters. ‘Erode’ comes in handy when fine-tuning the outline with minimal changes (1-2 pixels).

Ensure that alpha values remain within the 0 to 1 range for accurate compositing.

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Merge the output

• ‘Merge’ nodes are used to combine multiple elements. There are some ‘operation’ settings, like ‘plus’ and ‘stencil.’ For extracting roto, opt for the ‘stencil’ operation.
• Use ‘Channel Merge‘ or ‘Merge’ with the ‘operation’ set to ‘screen‘ to effectively combine multiple Roto nodes.(If you use ‘plus’ or another options, alpha could exceed 1, so it could ruin your compositing.)
• ‘Clamp’ node serves to establish minimum and maximum values for various node attributes, refining the final composition.